In general, the present invention relates to a method and apparatus for shimming a magnet. In particular, the present invention is a method for determining the placement of shim elements, and an apparatus used to place the shim elements in specific locations on the pole surface of a magnet in order to shape the magnetic field to achieve greater uniformity.
Magnetic resonance imaging (xe2x80x9cMRIxe2x80x9d) is one of the most versatile and fastest growing modalities in medical imaging. As part of the MRI process, the subject patient is placed in an external magnetic field. This field is created by a magnet assembly, which may be closed or open. Open magnet assemblies have two spaced-apart magnet poles separated by a gap, and a working magnetic field volume located within the gap.
The diagnostic quality of images produced by MRI is directly related to several system performance characteristics. One very important consideration is the uniformity, or homogeneity, of the main magnetic field. In order to produce high-resolution images, the magnetic field produced in the MRI scanner must be maintained to a very high degree of uniformity. However, an MRI magnet initially produces a field that is usually less uniform than that required to image successfully. At some point after manufacture, the magnet assembly must be adjusted to produce a more uniform field.
A process known as shimming is used to improve the homogeneity of the magnetic field to the necessary levels by making small mechanical and/or electrical adjustments to the overall field. Mechanical adjustments are called passive shimming, while electrical adjustments are known as active shimming. Electrical adjustments are effective because electrical current passing through a wire will produce a magnetic field around that wire. When these wires are formed into coils, the strength, direction, and shape of the magnetic field produced can be controlled by adjusting the physical and electrical parameters of the coils. Placing these coils in strategic locations as secondary magnetic field sources has the effect of adding to or subtracting from the main magnetic field in localized regions as well as over the entire pole surface, affecting the overall homogeneity of the main field. While the use of these xe2x80x9cshim coilsxe2x80x9d has allowed the homogeneity of the main MRI magnetic field to be greatly improved, there are numerous drawbacks associated with their use.
For example, the electric current in the shim coils may be unstable, resulting in an overall instability in the main magnetic field. This instability may cause xe2x80x9cghostingxe2x80x9d in the MR images. Ghosting is an interference phenomenon that appears at periodic intervals along the phase axis. These errors are unacceptable to any radiologist, who may confuse the correct position of the patient""s anatomic elements, possibly resulting in an incorrect diagnosis.
Further, shim coils are temperature sensitive. Variations in the temperature of the individual coils can cause instabilities in the main magnetic field, resulting in image artifacts. In addition, the currents used to produce the magnetic fields in the shim coils require complicated electronic circuits, such as voltage and current regulators and current amplifiers, to maintain stability. The shim coil can become inoperable when one or more of these electronic components breaks or goes out of tolerance. Even when all the electronic components are working properly, this type of active shimming adds expense and complexity to the overall MRI system. Passive shimming avoids adding complexity to the MRI system, but instead makes manufacture of the magnets more complicated, usually by requiring the custom physical modification of the magnet core components, such as shim bars, while adjusting the uniformity of the field produced by the newly-manufactured magnet.
It is common practice to represent the field in terms of a complete set of functions such as spherical harmonics, and to apply shims that attempt to reduce the amplitude of one of the functions. After one of the terms has been reduced to an acceptable level, the next term is considered. The problem with this method is that there may be many significant functions in an accurate representation of the field. To shim them one at a time is a long and tedious process.
Further, it is usually impossible to produce a shim that affects only a single higher order function such as a spherical harmonic. So instead of trying to shim the terms one at a time, the total field is shimmed all at once. Using particular modeling systems in which the approximation to the actual field is optimized when a large number of sequence terms is measured, inaccuracies result due to complex interdependencies and accumulated errors.
There is therefore a great need for a method of shimming a magnet to control the homogeneity of the resulting field that is accurate and efficient, in terms of both the time and the computational resources required to produce the desired homogeneity. The need also exists for an apparatus that may be used to effectively implement the method.
To overcome these and other disadvantages of the active (coil) shimming process, the present invention eliminates or minimizes the use of some or all shim coils and their associated currents altogether, achieving a high degree of field uniformity required for high resolution imaging through a process using only passive shimming. The shimming is effected through the use of metal shim elements that are added to the standard magnet in order to physically influence the overall field produced by the magnet. The shim elements may be held in place by a non-metallic cover that is affixed to the magnet. Preferably, a metal plate may be applied between the magnet and the non-metallic cover to incorporate a combination of shim elements in the form of additive metal elements, and other shim elements in the form of areas where metal has been removed from the metal plate. To overcome the disadvantages of conventional passive shimming techniques, the present invention provides a process for modeling the field and determining the physical parameters of the shim elements that is accurate and computationally straightforward.
According to an aspect of the present invention, a method for passively shimming a magnet includes measuring a magnetic field produced within a predetermined volume by the magnet, at selected points within the volume; modeling the measured magnetic field in the form of a plurality of additive components; detecting the degree of homogeneity in the measured field within the predetermined volume by examining measured magnetic field values at ones of the selected points corresponding to individual ones of the plurality of additive components; determining which additive component should be modified in order to change the homogeneity detected in the measured field; and coupling a metal element that provides the determined additive component modification when coupled to the magnet. Preferably, measuring the magnetic field includes producing magnetic field data for the measured points on a plane disposed with respect to a three-dimensional coordinate system, and modeling the measured magnetic field includes analyzing the magnetic field data and describing the analyzed data as the plurality of additive components, wherein each of the plurality of additive components has a particular symmetry with respect to the coordinate system. In a preferred embodiment, the plurality of additive components are eight additive components gi, such that for the field             f      ⁢              xe2x80x83            ⁢              (                  x          ,          y          ,          z                )              =                  ∑                  i          =          1                8            ⁢              xe2x80x83            ⁢              g        i              ,xe2x80x838g1=f(x, y, z)+f(xe2x88x92x, y, z)+f(x, xe2x88x92y, z)+f(xe2x88x92x, xe2x88x92y, z)+f(x, y, xe2x88x92z)+f(xe2x88x92x, y, xe2x88x92z)+f(x, xe2x88x92y, xe2x88x92z)+f(xe2x88x92x, xe2x88x92y, xe2x88x92z);
8g2=f(x, y, z)+f(xe2x88x92x, y, z)+f(x, xe2x88x92y, z)+f(xe2x88x92x, xe2x88x92y, z)xe2x88x92f(x, y, xe2x88x92z)xe2x88x92f(xe2x88x92x, y, xe2x88x92z)xe2x88x92f(x, xe2x88x92y, xe2x88x92z)xe2x88x92f(xe2x88x92x, xe2x88x92y, xe2x88x92z);
8g3=f(x, y, z)+f(xe2x88x92x, y, z)xe2x88x92f(x, xe2x88x92y, z)xe2x88x92f(xe2x88x92x, xe2x88x92y, z)+f(x, y, xe2x88x92z)+f(xe2x88x92x, y, xe2x88x92z)xe2x88x92f(x, xe2x88x92y, xe2x88x92z)xe2x88x92f(xe2x88x92x, xe2x88x92y, xe2x88x92z);
8g4=f(x, y, z)+f(xe2x88x92x, y, z)xe2x88x92f(x, xe2x88x92y, z)xe2x88x92f(xe2x88x92x, xe2x88x92y, z)xe2x88x92f(x, y, xe2x88x92z)xe2x88x92f(xe2x88x92x, y, xe2x88x92z)+f(x, xe2x88x92y, xe2x88x92z)+f(xe2x88x92x, xe2x88x92y, xe2x88x92z);
8g5=f(x, y, z)xe2x88x92f(xe2x88x92x, y, z)+f(x, xe2x88x92y, z)xe2x88x92f(xe2x88x92x, xe2x88x92y, z)+f(x, y, xe2x88x92z)xe2x88x92f(xe2x88x92x, y, xe2x88x92z)+f(x, xe2x88x92y, xe2x88x92z)xe2x88x92f(xe2x88x92x, xe2x88x92y, xe2x88x92z);
8g6=f(x, y, z)xe2x88x92f(xe2x88x92x, y, z)+f(x, xe2x88x92y, z)xe2x88x92f(xe2x88x92x, xe2x88x92y, z)xe2x88x92f(x, y, xe2x88x92z)+f(xe2x88x92x, y, xe2x88x92z)xe2x88x92f(x, xe2x88x92y, xe2x88x92z)+f(xe2x88x92x, xe2x88x92y, xe2x88x92z);
8g7=f(x, y, z)xe2x88x92f(xe2x88x92x, y, z)xe2x88x92f(x, xe2x88x92y, z)+f(xe2x88x92x, xe2x88x92y, z)+f(x, y, xe2x88x92z)xe2x88x92f(xe2x88x92x, y, xe2x88x92z)xe2x88x92f(x, xe2x88x92y, xe2x88x92z)+f(xe2x88x92x, xe2x88x92y, xe2x88x92z); and
8g8=f(x, y, z)xe2x88x92f(xe2x88x92x, y, z)xe2x88x92f(x, xe2x88x92y, z)+f(xe2x88x92x, xe2x88x92y, z)xe2x88x92f(x, y, xe2x88x92z)+f(xe2x88x92x, y, xe2x88x92z)+f(x, xe2x88x92y, xe2x88x92z)xe2x88x92f(xe2x88x92x, xe2x88x92y, xe2x88x92z);
wherein, y, and z are coordinates in the three-dimensional coordinate system.
According to another aspect of the invention, an apparatus for changing a homogeneity of a magnetic field produced by a magnet includes a first plate for placement near enough to the pole of the magnet so as to have an effect on the magnetic field, a shim for placement near enough to the first plate so as to have an effect on the magnetic field, and a second plate for attachment to the first plate such that the shim is held in place in a predetermined location between the first plate and the second plate. The shim corresponds to a magnitude change of an additive component of an equation describing the magnetic field, in order to change the homogeneity of the magnetic field. The additive component has a particular symmetry with respect to a three-dimensional coordinate system oriented with respect to a location of the pole. Preferably, the equation is             f      ⁢              xe2x80x83            ⁢              (                  x          ,          y          ,          z                )              =                  ∑                  i          =          1                8            ⁢              xe2x80x83            ⁢              g        i              ,
and the additive component is one of gi, i ={1,2,3,4,5,6,7,8}, wherein
8g1=f(x, y, z)+f(xe2x88x92x, y, z)+f(x, xe2x88x92y, z)+f(xe2x88x92x, xe2x88x92y, z)+f(x, y, xe2x88x92z)+f(xe2x88x92x, y, xe2x88x92z)+f(x, xe2x88x92y, xe2x88x92z)+f(xe2x88x92x, xe2x88x92y, xe2x88x92z);
8g2=f(x, y, z)+f(xe2x88x92x, y, z)+f(x, xe2x88x92y, z)+f(xe2x88x92x, xe2x88x92y, z)xe2x88x92f(x, y, xe2x88x92z)xe2x88x92f(xe2x88x92x, y, xe2x88x92z)xe2x88x92f(x, xe2x88x92y, xe2x88x92z)xe2x88x92f(xe2x88x92x, xe2x88x92y, xe2x88x92z);
8g3=f(x, y, z)+f(xe2x88x92x, y, z)xe2x88x92f(x, xe2x88x92y, z)xe2x88x92f(xe2x88x92x, xe2x88x92y, z)+f(x, y, xe2x88x92z)+f(xe2x88x92x, y, xe2x88x92z)xe2x88x92f(x, xe2x88x92y, xe2x88x92z)xe2x88x92f(xe2x88x92x, xe2x88x92y, xe2x88x92z);
8g4=f(x, y, z)+f(xe2x88x92x, y, z)xe2x88x92f(x, xe2x88x92y, z)xe2x88x92f(xe2x88x92x, xe2x88x92y, z)xe2x88x92f(x, y, xe2x88x92z)xe2x88x92f(xe2x88x92x, y, xe2x88x92z)+f(x, xe2x88x92y, xe2x88x92z)+f(xe2x88x92x, xe2x88x92y, xe2x88x92z);
8g5=f(x, y, z)xe2x88x92f(xe2x88x92x, y, z)+f(x, xe2x88x92y, z)xe2x88x92f(xe2x88x92x, xe2x88x92y, z)+f(x, y, xe2x88x92z)xe2x88x92f(xe2x88x92x, y, xe2x88x92z)+f(x, xe2x88x92y, xe2x88x92z)xe2x88x92f(xe2x88x92x, xe2x88x92y, xe2x88x92z);
8g6=f(x, y, z)xe2x88x92f(xe2x88x92x, y, z)+f(x, xe2x88x92y, z)xe2x88x92f(xe2x88x92x, xe2x88x92y, z)xe2x88x92f(x, y, xe2x88x92z)+f(xe2x88x92x, y, xe2x88x92z)xe2x88x92f(x, xe2x88x92y, xe2x88x92z)+f(xe2x88x92x, xe2x88x92y, xe2x88x92z);
8g7=f(x, y, z)xe2x88x92f(xe2x88x92x, y, z)xe2x88x92f(x, xe2x88x92y, z)+f(xe2x88x92x, xe2x88x92y, z)+f(x, y, xe2x88x92z)xe2x88x92f(xe2x88x92x, y, xe2x88x92z)xe2x88x92f(x, xe2x88x92y, xe2x88x92z)+f(xe2x88x92x, xe2x88x92y, xe2x88x92z); and
8g8=f(x, y, z)xe2x88x92f(xe2x88x92x, y, z)xe2x88x92f(x, xe2x88x92y, z)+f(xe2x88x92x, xe2x88x92y, z)xe2x88x92f(x, y, xe2x88x92z)+f(xe2x88x92x, y, xe2x88x92z)+f(x, xe2x88x92y, xe2x88x92z)xe2x88x92f(xe2x88x92x, xe2x88x92y, xe2x88x92z);
wherein the pole is disposed on the z-axis. In particular cases, the additive component is a first additive component having a first particular symmetry with respect to the three-dimensional coordinate system. In such a case, the first plate includes a grooved portion that corresponds to a magnitude change of a second additive component of the equation describing the magnetic field, in order to change the homogeneity of the magnetic field, and the second additive component has a second particular symmetry with respect to the three-dimensional coordinate system.
According to a further,aspect of the present invention, an apparatus for changing a homogeneity of a magnetic field produced by a magnet includes a shim and a plate for attachment to the magnet such that the shim is held in place in a predetermined location between the magnet and the plate. The shim corresponds to a magnitude change of an additive component of an equation describing the magnetic field, in order to change the homogeneity of the magnetic field. The additive component has a particular symmetry with respect to a three-dimensional coordinate system oriented with respect to a location of the pole. Preferably, the equation is             f      ⁢              xe2x80x83            ⁢              (                  x          ,          y          ,          z                )              =                  ∑                  i          =          1                8            ⁢              xe2x80x83            ⁢              g        i              ,
and the additive component is one of gi, i={1,2,3,4,5,6,7,8}, wherein
8g1=f(x, y, z)+f(xe2x88x92x, y, z)+f(x, xe2x88x92y, z)+f(xe2x88x92x, xe2x88x92y, z)+f(x, y, xe2x88x92z)+f(xe2x88x92x, y, xe2x88x92z)+f(x, xe2x88x92y, xe2x88x92z)+f(xe2x88x92x, xe2x88x92y, xe2x88x92z);
8g2=f(x, y, z)+f(xe2x88x92x, y, z)+f(x, xe2x88x92y, z)+f(xe2x88x92x, xe2x88x92y, z)xe2x88x92f(x, y, xe2x88x92z)xe2x88x92f(xe2x88x92x, y, xe2x88x92z)xe2x88x92f(x, xe2x88x92y, xe2x88x92z)xe2x88x92f(xe2x88x92x, xe2x88x92y, xe2x88x92z);
8g3=f(x, y, z)+f(xe2x88x92x, y, z)xe2x88x92f(x, xe2x88x92y, z)xe2x88x92f(xe2x88x92x, xe2x88x92y, z)+f(x, y, xe2x88x92z)+f(xe2x88x92x, y, xe2x88x92z)xe2x88x92f(x, xe2x88x92y, xe2x88x92z)xe2x88x92f(xe2x88x92x, xe2x88x92y, xe2x88x92z);
8g4=f(x, y, z)+f(xe2x88x92x, y, z)xe2x88x92f(x, xe2x88x92y, z)xe2x88x92f(xe2x88x92x, xe2x88x92y, z)xe2x88x92f(x, y, xe2x88x92z)xe2x88x92f(xe2x88x92x, y, xe2x88x92z)+f(x, xe2x88x92y, xe2x88x92z)+f(xe2x88x92x, xe2x88x92y, xe2x88x92z);
8g5=f(x, y, z)xe2x88x92f(xe2x88x92x, y, z)+f(x, xe2x88x92y, z)xe2x88x92f(xe2x88x92x, xe2x88x92y, z)+f(x, y, xe2x88x92z)xe2x88x92f(xe2x88x92x, y, xe2x88x92z)+f(x, xe2x88x92y, xe2x88x92z)xe2x88x92f(xe2x88x92x, xe2x88x92y, xe2x88x92z);
8g6=f(x, y, z)xe2x88x92f(xe2x88x92x, y, z)+f(x, xe2x88x92y, z)xe2x88x92f(xe2x88x92x, xe2x88x92y, z)xe2x88x92f(x, y, xe2x88x92z)+f(xe2x88x92x, y, xe2x88x92z)xe2x88x92f(x, xe2x88x92y, xe2x88x92z)+f(xe2x88x92x, xe2x88x92y, xe2x88x92z);
8g7=f(x, y, z)xe2x88x92f(xe2x88x92x, y, z)xe2x88x92f(x, xe2x88x92y, z)+f(xe2x88x92x, xe2x88x92y, z)+f(x, y, xe2x88x92z)xe2x88x92f(xe2x88x92x, y, xe2x88x92z)xe2x88x92f(x, xe2x88x92y, xe2x88x92z)+f(xe2x88x92x, xe2x88x92y, xe2x88x92z); and
8g8=f(x, y, z)xe2x88x92f(xe2x88x92x, y, z)xe2x88x92f(x, xe2x88x92y, z)+f(xe2x88x92x, xe2x88x92y, z)xe2x88x92f(x, y, xe2x88x92z)+f(xe2x88x92x, y, xe2x88x92z)+f(x, xe2x88x92y, xe2x88x92z)xe2x88x92f(xe2x88x92x, xe2x88x92y, xe2x88x92z);
wherein the pole is disposed on the z-axis. In particular cases, the additive component is a first additive component having a first particular symmetry with respect to the three-dimensional coordinate system. In such cases, the magnet includes a grooved portion that corresponds to a magnitude change of a second additive component of the equation describing the magnetic field, in order to change the homogeneity of the magnetic field, and the second additive component has a second particular symmetry with respect to the three-dimensional coordinate system.